A Glorious View

A layer of stratocumulus clouds over the Pacific Ocean served as the backdrop for this rainbow-like optical phenomenon known as a glory. Glories generally appear as concentric rings of color in front of mist or fog. They form when water droplets within clouds scatter sunlight back toward a source of illumination (in this case the Sun). TheModerate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired the image on June 21, 2012. The image was saturation-enhanced to make the glory effect more visible.

Although glories may look similar to rainbows, the way light is scattered to produce them is different. Rainbows are formed by refraction and reflection; glories are formed by backward diffraction. The most vivid glories form when an observer looks down on thin clouds with droplets that are between 10 and 30 microns in diameter. The brightest and most colorful glories also form when droplets are roughly the same size.

From the ground or an airplane, glories appear as circular rings of color. The space shuttle Columbia observed a circular glory from space in 2003. In the image above, however, the glory does not appear circular. That’s because MODIS scans the Earth’s surface in swaths perpendicular to the path followed by the satellite. And since the swaths show horizontal cross sections through the rings of the glory, the glory here appears as two elongated bands of color that run parallel to the path of the satellite, rather than a full circle.

Glories always appear around the spot directly opposite the Sun, from the perspective of the viewer. This spot is called the anti-solar point. To visualize this, imagine a line connecting the Sun, a viewer, and the spot where the glory appears. In this case, the anti-solar point falls about halfway between the two colored lines of the glory.

Glories are usually seen against a background of white clouds. Clouds are white because the sunlight is scattered many times by multiple droplets within the clouds. The white light often obscures details of glories, but without them in the background, the glory would not be visible.

Another notable feature in this image are the swirling von karman vortices that are visible to the right of the glory. The alternating double row of vortices form in the wake of an obstacle, in this instance the eastern Pacific island of Guadalupe.

A Disappearing Island Restored

Not so long ago, many islands rose above the brackish waters of the Chesapeake Bay. These small islands offered a predator-free haven for nesting water birds and turtles, while the larger islands supported fishing communities along with wildlife. But now, the muddy, marshy islands are eroding under the combined forces of geology and climate change. The very crust under the Chesapeake Bay is sinking, while sea levels are rising. Made of clay and silt, the islands erode quickly, and many have disappeared altogether.

Poplar Island ranks among those that would have been gone a decade ago if not for a massive restoration project. In the 1800s, the island had an area just over 1,000 acres and held a small town of about 100 people. By the 1990s, the island was nearly gone, containing a mere 10 acres of land. In the left image, taken by the Landsat 5 satellite on June 28, 1997, Poplar Island had been reduced to a tiny green dot surrounded by clouds of silt-laden water.

In 1998, the U.S. Army Corp of Engineers began to restore Poplar Island. The project serves two purposes: it restores lost habitat to birds and turtles, and it provides a use for material dredged from Baltimore Harbor and Chesapeake Bay shipping lanes. The method of restoration is visible in the center image, taken on June 21, 2006. Engineers built dikes around sections of the island and have been gradually filling in the center with dredged silt. By 2006, the island had regained the shape it held in the 1800s.

As each cell is filled with new soil, the Army Corp of Engineers plants vegetation. The right image, taken on July 5, 2011, shows that much of the island has been re-vegetated. Poplar Island now has an area of 1,140 acres and may continue to expand by another 500 acres before the restoration is completed in 2027. Upon completion, Poplar Island will be half wetlands and half uplands covered by forest. The restoration project is expected to cost $667 million, says the U.S. Army Corp of Engineers.

Islands and shorelines in the Mid-Atlantic may become increasingly vulnerable to erosion. Sea levels are rising as the ocean warms and expands—and as glaciers and ice sheets melt—but the rise isn't uniform around the planet. Currents, salinity, and topography create areas where sea levels are increasing more quickly, and recent researchfound that the U.S. Mid-Atlantic coast is one of the areas of accelerated sea-level rise. The rate of increase in the densely populated Mid-Atlantic is three to four times greater than average global sea-level rise. The increased sea level will make coastal regions and islands more prone to flooding and erosion.

A short animation of the Poplar Island restoration is available from the NASA Scientific Visualization Studio.

Sea Ice Retreat in the Beaufort Sea

As the summer solstice approaches in the Northern Hemisphere, long hours of sunlight warm the Arctic and melt snow and sea ice. Sea ice retreat in June is typical, but the first half of June 2012 brought unusually rapid ice loss, the National Snow and Ice Data Center (NSIDC) reported on June 19.

One area of rapid ice retreat was the Beaufort Sea, north of Alaska. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite collected these images on May 13, 2012 (top), and June 16, 2012 (bottom). By mid-June, the open-water area off the coast had expanded substantially and snow had melted on land.

The rapid melt north of Alaska was part of a larger phenomenon. Sea ice across the entire Arctic reached record-low levels for this time of year, NSIDC stated, slightly below the previous record set in June 2010. It was also lower than the extent in June 2007; Arctic sea ice reached its lowest extent ever recorded by satellite in September 2007.

In the first half of June 2012, the Beaufort Sea was a “hotspot” of rapid retreat, driven by a high-pressure pattern over the region that kept skies clear at the very time of year when sunlight lasts the longest. In addition, larger-scale climate patterns in early June 2012 favored ice retreat along the coastlines of Alaska and Siberia. As of June 18, temperatures were above freezing over much of the sea ice in the Arctic, and snow had melted earlier than normal, leading to warming on land.

On June 19, 2012, NSIDC reported: “Recent ice loss rates have been 100,000 to 150,000 square kilometers (38,600 to 57,900 square miles) per day, which is more than double the climatological rate.” (For comparison, the area of the state of Illinois is roughly 150,000 square kilometers.)

The early onset of the spring melt and the sunny skies around the solstice increased the likelihood of heightened melt rates throughout the rest of the summer, largely by reducing albedo: the proportion of solar energy reflected back into space. If an object reflects all the energy it receives, it has an albedo of 1.0. Sea ice has high albedo because of its bright appearance. But when it starts to melt, its albedo drops from roughly 0.9 to 0.7, causing the ice to absorb more energy. Increased energy absorption leads to increased melt, which exposes ocean water. Thanks to its dark appearance, ocean water has an albedo of less than 0.1. Long, sunny days pour energy into the water, and it retains the heat throughout the summer. In September, when the Sun is low on the horizon, the heated ocean water continues melting sea ice.

1.   References

2.   NSIDC. (2012, June 19) Sea ice tracking at record low levels. Accessed June 22, 2012.

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Waldo Canyon Fire Burn Scar

The Waldo Canyon Fire was first reported on June 23, 2012, burning in Pike National Forest, three miles (5 kilometers) west of Colorado Springs. Fueled by extremely dry conditions and strong winds, it had burned 18,247 acres (74 square kilometers) by July 5. The blaze severely damaged or destroyed 346 homes, making it the most destructive fire in Colorado history. Mountain Shadows, a neighborhood northwest of the Colorado Springs city center, experienced some of the most severe damage. According to an analysis conducted by the Denver Post, the combined value of the homes that burned to the ground in the neighborhood was at least $110 million.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on the Terra satellite acquired this view of the burn scar on July 4, 2012, when the fire was still burning but was 90 percent contained. Vegetation-covered land is red in the false-color image, which includes both visible and infrared light. Patches of unburned forest are bright red, in contrast with areas where flecks of light brown indicate some burning. The darkest brown areas are the most severely burned. Buildings, roads, and other developed areas appear light gray and white. The bright red patches of vegetation near Colorado Springs are golf courses, parks, or other irrigated land.

·      References

·      Inciweb. (2012, July 5). Waldo Canyon Fire. Accessed July 5, 2012.

·      Denver Post. (2012, July 4). Colorado Wildfire: Waldo Toll Hits $110 Million for Lost Homes. Accessed July 5, 2012.

·      Colorado Springs Fire Department. (2012, July 3). Waldo Canyon Fire. Mountain Shadows (Video). Accessed July 5, 2012.

1.     Further Reading

2.     Gazette.com. (2012, July 3). Meeting Thursday Night to Address Flood Concerns. Accessed July 5, 2012.